The atmosphere cooled and thinned. The pressure dropped.

The ocean froze over.

New methane lakes formed, which converted slowly to ethane. Sunlight broke up atmospheric ammonia, to release a new atmosphere of nitrogen.

The moon settled into its long freeze.

But it was not inert. Ultraviolet photons from the sun and charged particles trapped in Saturn’s magnetic field beat down on the thick layer of air. Chemistry continued in the new atmosphere, and complex organic deposits rained down on the frozen surface.

Thus, for billions of years, Titan waited.

An object looking a little like a comet streaked across the sky of Titan, battering atmospheric gases to a plasma twice as hot as the surface of the sun itself.

Cooling, it fell towards the surface slush.

A parachute blossomed above it.

Huygens was built like a shellfish, with a tough outer cover shielding a softer kernel, with its fragile load of instrumentation. When its job was done, the outer aeroshell broke open, like the two halves of a clam shell, and the main chute unfolded.

So, after being carried across a billion miles, the aeroshell was discarded. It had absorbed nearly a third of the probe’s entire mass.

The descent module, exposed, was built around a disc-shaped platform of thick aluminum. Experiments and probe systems were bolted to the platform. The equipment was shrouded by a shell of aluminum, with a spherical cap for a nose and a truncated cone for a tail. It looked something like an inverted clam. Now cutouts in the shell opened, and booms unfolded from the main body. Instruments peered through the cutouts, or were held mounted on the booms, away from the main body.